Removal of NG on existing housing would have no impact on rents, properties sold by investors still exist, they do not vanish. This is a common myth pushed by vested interests.
Yes most speculators are active in the fhb price bracket since that's where the higher yields are. Won't be feasible really though to restrict ng to certain price brackets, a total phase out on existing properties is what will happen.
NG has heavily impacted first home buyers by encouraging speculators instead and pushing up prices. The sooner it goes the better, now is an excellent opportunity with low interest rates reducing the impact of its removal.